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C:\Users\[USERNAME]\AppData\Local\Packages\Microsoft.Windows.ContentDeliveryManager\LocalState\Asset
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C:\Windows\Web\Screen
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C:\Windows\Web\Wallpaper
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C:\ProgramData\Microsoft\Windows\SystemData
EXPLANATION
C:\Users\[USERNAME]\AppData\Local\Packages\Microsoft.Windows.ContentDeliveryManager\LocalState\Asset
is the correct answer... given 100 characters to work with. Since the
backgrounds are downloaded, stored and overwritten.. they are stored
here.
C:\ProgramData\Microsoft\Windows\SystemData is where the lock screen can be changed or overwritten, given permission.
C:\Windows\Web\Screen is the location of the windows prelogin screen background.
And
C:\Windows\Web\Wallpaper is where default windows 7 wallpapers for your desktop are saved.
SOURCE
https://www.laptopmag.com/articles/find-windows-10-lock-screen-pictures